The present invention relates to coupled cavity type traveling wave tubes and, more particularly, to an improved pole piece structure for the coupled cavity type traveling wave tube.
The traveling wave tube is a known type of electronic vacuum tube device useful as a component element of microwave electronic systems, such as radar systems, as an amplifier of microwave frequency energy. In its structure and operation, the device operates upon the physical principle of electronic interaction between an electron beam and a microwave frequency signal, which is made to propagate along a slow wave structure of the device, to transfer energy from the electron beam to the microwave frequency signal. The slow wave structure provides the means by which the propagating microwave signal may be caused to traverse a greater length between two axially spaced points so that the effective lateral propagation velocity is reduced or slowed from that of the velocity of light, at which such microwave energy propagates, to the lower velocity of the electrons in the electron beam, a relationship necessary to accomplish the phenomenon of an energy transferring electronic "interaction" between the electron beam and the microwave signal.
The present invention is concerned with traveling wave tubes utilizing slow wave structures of the coupled cavity or interconnected cell type, as variously termed. In this type of slow wave structure, a plurality of interaction cells or cavities are disposed serially and adjacent one another along a common axis. A plurality of axially aligned passages through the cavities is provided for passage of an electron beam. Further, each interaction cavity in the plurality is coupled to an adjacent cavity by means of a coupling aperture in an end wall. Generally the coupling aperture between adjacent cavities are alternately disposed on opposite sides of the electron beam axis, although other arrangements and variations are possible. An electron gun containing a cathode located within the tube furnishes a source of electrons which are formed into a beam and directed along a straight path through the aforedescribed passages in the cavities. Electronic interaction occurs between the electron beam and the microwave frequency signal appearing at the cavity proximate the beam.
Magnetic means are used with the tube to provide magnetic fields that function to confine or focus the electron beam to the axial path so as to minimize electron beam spreading, as hereinafter discussed. This is accomplished in the traveling wave tube by utilizing an iron material for the cavity end wall in the aforedescribed slow wave structure, commonly termed a pole piece, and permanent magnets located outside of the vacuum region of the tube provide the magnetic flux coupled to the pole pieces. Typically, these pole pieces have protruding lips or "ferrules" projecting from the front and back sides of the pole piece walls. These portions surround the electron beam passage to provide a concentrated axially extending magnetic field between the ferrule of one pole piece and that of an adjacent pole piece. Secondly, the beam passage so formed in the pole piece between the ends of these lip portions functions as a "drift tube" region.
Although the preceding background is necessarily brief, the interested reader may make reference to various patents in the patent literature for a more detailed understanding of the structure and operation of the coupled cavity type traveling wave tube to which reference is made as follows:
(1) U.S. Pat. No. 2,985,792, issued May 23, 1961, D. J. Bates; PA1 (2) U.S. Pat. No. 3,989,978, issued Nov. 2, 1976, Sauseng et al; PA1 (3) U.S. Pat. No. 3,221,204, issued Nov. 30, 1965, Hant et al; PA1 (4) U.S. Pat. No. 3,181,023, issued Apr. 27, 1965, Hant et al; PA1 (5) U.S. Pat. No. 3,010,047, issued Nov. 21, 1961, Bates; PA1 (6) U.S. Pat. No. 3,602,766, issued Aug. 31, 1971, Grant; PA1 (7) U.S. Pat. No. 3,324,339, issued June 6, 1967, Winslow et al.
As is true in most devices, the operation of the traveling wave tube differs in practice somewhat from theory and is subject to various practical limitations which limit the extent of operation of the device. Thus despite the fact that the aforedescribed magnetic focusing structure is intended to fully confine the electron beam to the maximum diameter of the beam passages, it is not possible to do so completely. Thus, in operation of the tube, when the electron beam is generated, some of the electrons from the beam in fact do strike the pole piece walls. The high kinetic energy released by electrons traveling at a high velocity functions disadvantageously to heat the pole piece and may cause either warping, which disturbs the operation of the tube, or melting of the pole piece material. At increased or higher "duty cycles", i.e. when the electron beam is "on" for longer periods of time, in which higher density electron beams are used in the operation of the tube, the amount of power dissipated at the pole piece by those "errant" electrons becomes quite large and absent any provision for cooling that tube element the power level at which the pole piece warpage or damage occurs is obviously a limitation of the power level at which the traveling wave tube is capable of properly operating. While being unable presently to provide effective tube structure to further reduce or eliminate such electron impingement, various efforts have been undertaken with structure to minimize the heating effect caused by those impinging electrons by the provision of cooling or heat dissipating structures.
Two heat dissipating structures which have been made known to me for providing cooling in this type of traveling wave tube are, first, a pole piece structure fabricated with minute coolant channels therein. Such cooling channels extend from the outer periphery of the pole piece down to the area of the electron beam passage and the ferrules. Additional means are provided which permit the coolant channels to extend outside the tube to a coolant source. In this way, coolant fluid may be pumped into the pole piece and heat is transferred from the pole piece to the fluid, and, in turn, the heated fluid is passed from the traveling wave tube to a heat "sink". This method, as I understand, is very effective as well as flexible, possessing as a drawback the very expensive fabrication techniques used in the construction of the pole pieces themselves. The second cooling structure, as has been related to me, for accomplishing heat dissipation without the use of a fluid coolant source or channels, is one in which strips or layers of copper material which, as is known, has a higher coefficient heat transfer characteristic than the iron pole piece material, is affixed or laminated to each of the front and back side of the iron pole piece. In this way, heat may be conducted from the area of the electron beam passage and pole piece ferrule to the outer periphery of the traveling wave tube where it can be air or liquid cooled.
The aforedescribed prior art structure, in my opinion, presents some limitations in application and use. First, the copper patches were machined to the proper size before they were brazed at high temperature to the iron pole piece material. And because the thermal expansion characteristic of each of the copper and iron are different, it was difficult to braze the patches uniformly and completely to the iron resulting in some warpage of the pole piece. Secondly, in the operation of the traveling wave tube containing those pole pieces any heat generated through electron bombardment of the electron beam passage walls or ferrules is necessarily conducted through the iron of the pole piece to the copper patches, which I view as a less than fully effective heat transfer path.
The present invention is concerned with the dissipation of heat energy caused by electron impingement on the magnetic pole pieces and has as an object an improved tube structure which permits traveling wave tube operation at high power levels without the use of expensive coolant channels or unreliable copper patches. A further object of the invention is to provide a pole piece structure of improved construction capable of rapidly dissipating or transferring heat generated in the pole piece ferrule area about the electron beam passage, which avoids localized heating, results in minimal pole piece warpage and is relatively inexpensive to manufacture. As a further object of the invention, there is provided a new process for fabricating a pole piece structure for a low frequency coupled cavity type traveling wave tube.